Abstract
Hydraulic fracturing (HF) is widely used in shale gas development, which may cause some heavy metals release from shale formations. These contaminants could transport from the fractured shale reservoirs to shallow aquifers. Thus, it is necessary to assess the impact of pollution in shallow aquifers. In this paper, a new analysis model, considering geological distributions, discrete natural fractures (NFs) and faults, is developed to analyze the migration mechanism of contaminants. Furthermore, the alkali erosion of rock caused by high-pH drilling of fluids, is considered in this paper. The numerical results suggest that both NFs and alkali erosion could reduce the time required for contaminants migrating to aquifers. When NFs and alkali erosion are both considered, the migration time will be shortened by 51 years. Alkali erosion makes the impact of NFs, on the contaminant migration, more significant. The migration time decreases with increasing pH values, while the accumulation is on the opposite side. Compared with pH 12.0, the migration time would be increased by 45 years and 29 years for pH 11.0 and 11.5, respectively. However, the migration time for pH 12.5 and 13.0 were found to be decreased by 82 years and 180 years, respectively. Alkali erosion could increase the rock permeability, and the elevated permeability would further enhance the migration velocity of the contaminants, which might play a major role in assessing the potential contamination of shallow aquifers.
Highlights
With the rapid decline in conventional reserves and increase in energy demand, hydraulic fracturing (HF) has emerged as an attractive technology for extracting unconventional resources from low permeability shale reservoirs [1,2,3]
Fluid flow in porous media is very important in a wide range of science and engineering applications, such as shale gas extraction and contaminant migration [24]
A new analysis model, considering the heterogeneity of the subsurface and the alkali erosion of rock, was chosen as an application example to analyze the effect of the natural fractures (NFs) and the alkali erosion, on the aquifer’s pollution levels
Summary
With the rapid decline in conventional reserves and increase in energy demand, hydraulic fracturing (HF) has emerged as an attractive technology for extracting unconventional resources from low permeability shale reservoirs [1,2,3]. There are four kinds of contaminants during shale gas extraction: Formation fluid, flow-back fluid, drilling fluid, and HF fluid. Some investigations [6,7] have shown that a number of heavy metals release from shale formations during gas extractions. Shows that flow-back liquid contains some heavy metals. Among these heavy metals, cobalt and zinc are essential nutrients, but can be toxic in larger amounts or certain forms. It is urgent to investigate the migration mechanism of heavy metal contaminants in a heterogeneous deep subsurface, considering the alkali erosion of rock
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